Implantable valve prosthesis and method for manufacturing such a valve

ABSTRACT

The invention relates to an implantable valve prosthesis, optionally comprising a support structure and at least one valve leaflet. The valve prosthesis comprises a material structure of unidirectional reinforcing elements of drawn ultra high molecular weight polyolefin, extending in at least two directions, wherein the modulus of elasticity of the polyolefin reinforcing elements is at least 60 GPa. The invention furthermore relates to a method for manufacturing such an implantable valve. The valve is easily manufactured and has an improved durability.

The invention relates to an implantable valve prosthesis and a methodfor manufacturing such an implantable valve prosthesis.

Implantable valve prostheses may for instance be used as aone-direction-valve prosthesis in the human or animal body, especiallywithin or nearby blood vessels. A suitable valve prosthesis needs toopen readily, cause no or only a little turbulence in the blood flow,and avoid regurgitation.

An aging population and increasing welfare lead to a higher demand forcardiovascular surgeries and their related products. Also, humans tendto demand more from their body since they live longer and performincreased physical exercise. Cardiovascular products, such as heartvalve prostheses, are thus subject to increased requirements withrespect to loading conditions, both in magnitude as in number of cycles.Typically, heart valve leaflets may undergo over a billion load cyclesin their lifetime. Durability is therefore an important requirement.

An implantable valve prosthesis is described in NL1008349. The valveprosthesis of NL1008349 comprises a support structure carrying a numberof valve leaflets. The valve leaflets are made by winding reinforcingfibers onto a mandrel in specific directions, as required by theoccurring stresses in the leaflets. Since the fibers have to bepositioned exactly according to the maximum stress lines, the knownvalve prosthesis is difficult to make and uses many wound layers toaccommodate stresses, whereby mass is added. Moreover the known valveprosthesis can be improved further with respect to its durability,particularly in view of the increased requirements imposed onimplantable valve prostheses nowadays.

Another implantable valve prosthesis is known from U.S. Pat. No.6,726,715. As in NL1008349, the disclosed valve prosthesis comprisesleaflets that are reinforced with fibers oriented along lines of stressin the leaflets. In this way, local variations of stress are accountedfor. The valve prosthesis according to U.S. Pat. No. 6,726,715 suffersfrom the same disadvantage as the valve prosthesis of NL1008349, amongothers in that it is difficult to make.

WO 2004/032987 concerns a medical device having at least three layers ofpolymeric components arranged in a sandwich construction of which thepolymeric component of the middle layer has a shorter chain length thanthe other polymeric components. A heart valve is mentioned as onepossible application of the sandwich construction.

The object of the present invention is therefore to provide animplantable valve prosthesis that is readily manufactured and yet showsequal or improved durability over the prior art. Another object of thepresent invention is to provide a method for manufacturing such animproved valve prosthesis.

This object is achieved by providing an implantable valve prosthesis,comprising at least one valve leaflet and optionally a support structurefor the at least one valve leaflet, the valve prosthesis being made of amaterial structure of unidirectional reinforcing elements of drawn ultrahigh molecular weight polyolefin, extending in at least two directions,wherein the modulus of elasticity of the polyolefin reinforcing elementsis at least 60 GPa. By using the described ‘two-directional’ materialstructure, the valve prosthesis according to the invention is easilymanufactured. Moreover it exhibits improved durability. This issurprising since it is well known that a high modulus of elasticitygenerally is associated with more brittle materials, thus yieldsunfavourable durability, in particular fatigue properties, due to thehigher stress levels incurred.

By ‘two-directional’ is herein meant that the reinforcing elements areoriented in at least two directions for example at least two directionsin a (curved) plane or surface or arranged in another three dimensionalstructure optionally in a matrix material, such as in two, three, four,five, six, seven, eight or even more directions.

The modulus of elasticity of the polyolefin is measured according to thestandard ASTM D2256. The modulus measured, is the initial modulus asdescribed in section 16 of ASTM D2256. Preferably the modulus ofelasticity of the polyolefin reinforcement element is at least 65 GPa,More preferably at least 80 GPa. The maximum modulus is rather limitedby the production technology of the polyolefin elements than byapplicability in the valve prosthesis. So far, upper modulus limits formaking superior valves are not encountered.

The valve prosthesis according to the invention comprises a materialstructure of unidirectional reinforcing elements extending in at leasttwo directions. Such material structures may be produced whilemanufacturing the valve prosthesis or may, preferably, be producedbeforehand as a semi-finished product. The valve prosthesis according tothe invention is in particular advantageous since it allows using suchrelatively simple material structures, yet shows a good durability. Easymanufacturing is thus combined with long duration performance.

Since the valve prosthesis according to the invention preferably showsan increased fatigue strength over the prior art, thinner valve leafletsmay be designed for similar fatigue strength in particular applications,such as in blood vessels. This saves additional mass.

In a first embodiment of the valve prosthesis according to the inventionthe thickness of the reinforcing elements is less than 40 micrometer,more preferably less than 30 micrometer and most preferably less than 20micrometer. A valve prosthesis according to this preferred embodimentshows a combination of good deformability and fatigue resistance.Another advantage of the use of reinforcing elements with low thicknessin the valve, and in particular in the valve leaflets, is that a highsurface smoothness may be achieved, which benefits a low level ofturbulence around the valve, and therefore diminishes the risk for bloodcoagulation.

In a preferred embodiment, the material structure comprises a pluralityof superimposed multilayered material sheets, the sheets comprising aconsolidated stack of monolayers of drawn ultra high molecular weightpolyolefin, whereby the draw direction of two subsequent monolayers inthe stack differs. It should be observed that this does not exclude thesituation where some subsequent monolayers have the same draw directionas long as some of the (sets of) adjacent monolayers do have differentdraw direction. A stack of five layers where layer 3 and 4 have the samedraw direction and this draw direction differs from layer 2 would hencebe an embodiment of the invention (irrespective of the orientation oflayer 1 and 5). However, in a preferred embodiment, most of the (setsof) subsequent monolayers have different draw direction, such as morethan 50%, preferably more than 75% and most preferably more than 90%.With monolayers in the context of this application is meant layerscomprising a plurality of unidirectional reinforcing elements of thedrawn polyolefin, aligned in the same direction.

In another preferred embodiment the material structure comprises atleast one material sheet, the at least one material sheet comprising awoven fabric of the drawn reinforcing elements. In yet another preferredembodiment, the material structure comprises a knitted braid of thedrawn reinforcing elements. The fiber directions in a braid can beeasily tuned by axial elongation and tend to readily accommodate thestresses in the leaflets, in particular in the leaflets of tricuspidvalve prostheses. Furthermore, it was found that the handleability ofbraided and woven fabric is better than the handleability ofunidirectional monolayers leading to a more flexible and affordablemanufacturing processing.

In a highly preferred embodiment, the material structure consist of onewoven fabric or knitted braid of the reinforcing elements of drawn ultrahigh molecular weight polyolefin. The woven fabric may for example be a3D weaving or braiding forming a mechanically interlocked unitedstructure having unidirectional reinforcing elements arranged in atleast two directions. The material structure may also for example bewoven non-woven, knitted, braided or weaved structure (for example anumber of UHMWPE UD tapes knitted together) or a material structureformed by any combination of these techniques, which creates a unitedstructure. This is highly preferred, as it prevents or strongly reducedthe risk of delaminating of a multilayered structure.

It is highly preferred that the structure does not comprise a sandwichstructure with a soft material arranged between layers of UHMWPE, as itwas found that this tend to lead to delamination of the valveconstruction due to failure of the soft layer or failure of theinterface between the soft layer and one of the UHMWPE layers.

It is advantageous when the material structure comprises a plurality ofdrawn polyolefinic reinforcing elements embedded in a matrix binder. Thebinder may enclose the polyolefin reinforcing elements in their entiretyor in part, such that the material structure is retained during handlingand manufacturing of the valve leaflets. The binder may be applied invarious forms or ways, for example as a film, that is melted to at leastpartially cover the polyolefin reinforcing elements, as a transversebonding strip or as transverse reinforcing elements, or by impregnatingand/or embedding the reinforcing elements with a polymer matrix in theform of a melt, a solution or a dispersion of a polymeric material in aliquid. In a preferred embodiment the binder is a polymeric matrixmaterial, and may be a thermosetting material or a thermoplasticmaterial, or a mixture of the two. If a thermosetting material isutilized, the thermosetting material is preferably at least partiallycured and preferably fully cured in the final product. A matrix bindermay also be added as a fibre with a lower melting point than thereinforcements in the fabric, hence allowing for melting and at leastpartially wetting of the UHMWPE during shaping of the valve. Theelongation at break of the matrix material is preferably greater thanthe elongation of the polyolefin fibers. In case the matrix material isa thermosetting polymer, vinyl esters, unsaturated polyesters, epoxiesor phenol resins are preferably selected. In case the matrix material isa thermoplastic polymer, polyurethanes, polyvinyls, polyacrylics,polyolefins and/or thermoplastic elastomeric block copolymers such aspolyisoprene-polyethylene-butylene-polystyrene orpolystyrene-polyisoprene-polystyrene block copolymers are preferablyselected. Preferably, the amount of binder in the monolayers is at most30% by weight, more preferably at most 25, 20 or even 15% by weight.

The reinforcing elements of the material structure may comprise fibres,unidirectional oriented tapes, bi-directional or multi directionaloriented tapes, films or combinations of these. With unidirectionaltapes or films is meant in the context of this application tapes orfilms, which show a preferred orientation of the polymer chains in onedirection, i.e. in the direction of drawing. Such tapes and films may beproduced by drawing, preferably by uniaxial drawing, and will in thatcase exhibit anisotropic mechanical properties.

The elements can be made with various technologies. Three technologiesare presented in the following.

The first preferred technology is by melt processing of fibrouselements. A linear polyethylene with a number averaged molecular mass(Mn) between 30 000 and 700 000 should be preferably be chosen and aratio between the weight averaged molecular mass (Mw) and Mn, shouldpreferably not exceed a value of 2. So preferably Mw/Mn<2. in case Mn isin the lower range, a higher ratio can be accepted. However, for makingstrong elements, the upper range is preferred. Higher strength valuesare possible with high Mn values and low Mw/Mn ratios are then necessaryin view of processing. Optimal conditions can be found by mereexperimentation. After melting in an extruder, the material is fedthrough a set of orifices and a first elongation is achieved by drawingin the melt stage. Subsequent drawing of the fibers at increasingtemperatures, but still below the melting point increases the tensilestrength.

A second more preferred technology is by solid state processing ofso-called disentangled polymers. An examples of such disentangledpolymers is presented in WO93/1518 (incorporated herein by reference).An example of processing of such powders to strong members is presentedin EP1627719 from p. 7 line 41 to p. 8. line 15 (incorporated herein byreference). A difference between EP1627719 and the present members isthat the members in EP1627719 are for ballistic purposes. The width forthe present application would be chosen to be much smaller. However,essential changes in the process are not necessary.

A third and the most preferred technology is so-called gel spinning.Many publications are available on this. An example is WO2005/066401(incorporated herein by reference).

Drawing, preferably uniaxial drawing, of the produced films may becarried out by means known in the art. Such means comprise extrusionstretching and tensile stretching on suitable drawing units. Thestretching may also be biaxial or multi axial—particularly when thestretched member is not unidirectional. To attain increased mechanicalstrength and stiffness, drawing may be carried out in multiple steps.

The strength of the reinforcing elements in the material structuredepends amongst others on the polyolefin from which they are produced,the production method of the fiber and on their (uniaxial) stretchratio. The tensile strength of the reinforcing elements measured in thedirection of the fiber is preferably at least 0.9 GPa, more preferablyat least 1.2 GPa, even more preferably at least 1.5 GPa, even morepreferably at least 1.8 GPa, even more preferably at least 2.1 GPa, andmost preferably at least 3 GPa. The strength of the material structurewill generally depend on the volume fraction of the reinforcing elementsin the material structure.

In yet another preferred embodiment the valve prosthesis according tothe invention is substantially free of matrix binder. Such an embodimentis more effective, having the same mechanical performance for lessmaterial. Moreover this embodiment improves blood compatibility. If nomatrix binder is used, the polyolefin fibers may preferably be partiallyfused during the manufacturing of the final product.

The fibres, unidirectional oriented tapes and films preferably compriseultra high molecular weight polyethylene. The ultra high molecularweight polyethylene may be linear or branched, although preferablylinear polyethylene is used. Linear polyethylene is herein understood tomean polyethylene with less than 1 side chain per 100 carbon atoms, andpreferably with less than 1 side chain per 300 carbon atoms; a sidechain or branch generally containing at least 10 carbon atoms. Sidechains may suitably be measured by FTIR on a 2 mm thick compressionmoulded film, as mentioned in e.g. EP 0269151. The linear polyethylenemay further contain up to 5 mol % of one or more other alkenes that arecopolymerisable therewith, such as propene, butene, pentene,4-methylpentene, octene. Preferably, the linear polyethylene is of highmolar mass with an intrinsic viscosity (IV, as determined on solutionsin decalin at 135° C.) of at least 4 dl/g; more preferably of at least 8dl/g, most preferably of at least 10 dl/g. Such polyethylene is alsoreferred to as ultra high molecular weight polyethylene, UHMWPE. In caseof the preferred ultra high molecular weight polyethylene films, drawingis typically carried out uniaxially in a number of drawing steps. Thefirst drawing step may for instance comprise drawing to a stretch factorof 3. Multiple drawing may typically result in a stretch factor of 9 fordrawing temperatures up to 120° C., a stretch factor of 25 for drawingtemperatures up to 140° C., and a stretch factor of 50 for drawingtemperatures up to and above 150° C. By multiple drawing at increasingtemperatures, stretch factors of about 50 and more may be reached. Thisresults in high strength tapes, whereby for tapes of ultra highmolecular weight polyethylene, strengths more than 3 GPa may beobtained.

The thickness of the material structure, and in particular of thematerial structure in the valve leaflets, may be varied within largeranges. However, preferably the thickness of the material structureranges between 50 and 500 micrometer, more preferably between 80 and 400micrometer and most preferably between 150 and 250 micrometer. Suchthickness allows for effective closing and opening of the valve leafletswith minimal effort and minimal regurgitation (blood flow in a nondesired direction), while maintaining sufficient durability.

In a preferred embodiment at least one valve leaflet is integrallyformed with at least a part of the support structure. Using a materialstructure in the form of a semi-finished product comprising reinforcingelements extending in at least two directions allows to readilyintegrate structures with different design requirements. Integratingsupport structure and leaflets establishes a durable connection betweenthe two.

In another preferred embodiment the support structure (also referred toas stent) of the valve prosthesis according to the invention comprises arigid member, and more preferably is ring shaped. Suitable materials formaking the support structure include rigid polymers, fibre reinforcedpolymers, metals and their alloys, ceramics and combinations thereof.

Suitable rigid polymers include polyacetals such as Delrin® and Celcon®,dextroplast, polyurethane, polyethylene, rubber, polysulfones,polyethersulfones, polyarylsulfones, polyetheretherketones, andpolyetherimides. Suitable metals include biocompatible metals, such as,stainless steel, titanium, cobalt alloys, such as Elgiloy®, acobalt-chromium-nickel alloy, and MP35N, anickel-cobalt-chromium-molybdenum alloy, and Nitinol®, a nickel-titaniumalloy. In addition, stents can be produced from ceramic materials, suchas pyrolytic carbon, silicon carbides or metal carbides, hydroxyapatiteand alumina. Suitable stents can also be produced from carbons such asgraphite.

Preferably, the support structure is at least partly made from a superelastic alloy, or a shape memory alloy, such as Nitinol®, anickel-titanium alloy, that is available as a super elastic material, aswell as a shape memory alloy. Such a support structure allows to easilyinsert the valve prosthesis into the body in a desired position. Beforeinsertion, the support structure is brought to a first (relatively low)temperature at which is has a compact configuration. This compactconfiguration allows to easily insert the support structure (and theleaflets) into the body, using minimal invasive surgery. Afterpositioning the support structure, and due to the body temperature, theshape memory alloy will heat up and change phase, thereby changing itsshape. For Nitinol® for instance, a phase change will occur between anaustenitic phase and a martensitic phase. As a result the supportstructure will expand and thereby creates a clamping force againstsurrounding tissue. In another configuration, Nitinol® is super elasticand can be deformed up to material strains of about 10%, thusdeformation of a valve towards a compact shape is possible, stillallowing elastic deployment to the final shape after placement.

The invention also relates to a process for the manufacture of animplantable valve prosthesis, the process comprising the steps of:

-   -   (a) providing a material structure of unidirectional reinforcing        elements of drawn ultra high molecular weight polyolefin,        extending in at least two directions, wherein the modulus of        elasticity of the polyolefin reinforcing elements is at least 60        GPa;    -   (b) shaping the material structure in the form of at least a        part of the valve prosthesis;    -   (c) consolidating the material structure under elevated        temperature and pressure.

Preferably, the process steps are applied in the given order, as thisallows for accurate individual production of the valve prostheses.However this is not necessary and process steps may be reversed. Forinstance, it is also possible to first consolidate the materialstructure and thereafter shape at least part of the valve prosthesis byfor instance taking away surplus material.

Consolidation may suitably be done in a hydraulic press. Consolidationis intended to mean that (at least part of) the material structure isrelatively firmly cohered to form one unit. The temperature duringconsolidating generally is controlled through the temperature of thepress. A minimum temperature generally is chosen such that a reasonablespeed of consolidation is obtained. In this respect 80° C. is a suitablelower temperature limit, preferably this lower limit is at least 100°C., more preferably at least 120° C., most preferably at least 140° C. Amaximum temperature is chosen below the temperature at which the drawnpolyolefin reinforcing elements lose their high mechanical propertiesdue to e.g. melting. Preferably the temperature is at least 5° C.,preferably at least 10° C. and even more preferably at least 15° C.below the melting temperature of the drawn polyolefin reinforcingelements. In case the drawn polyolefin reinforcing elements do notexhibit a clear melting temperature, the temperature at which the drawnpolyolefin reinforcing elements starts to lose their mechanicalproperties should be read instead of melting temperature. In the case ofthe preferred ultra high molecular weight polyethylene, a temperaturebelow 149° C., preferably below 145° C. generally will be chosen. Thepressure during consolidating preferably is at least 7 MPa, morepreferably at least 15 MPa, even more preferably at least 20 MPa andmost preferably at least 35 MPa. The optimum time for consolidationgenerally ranges from 5 to 120 minutes, depending on conditions such astemperature, pressure and thickness and can be verified through routineexperimentation.

Preferably, cooling after compression moulding at high temperature iscarried out under pressure as well. Pressure is preferably maintained atleast until the temperature is sufficiently low to prevent relaxation.This temperature can be established by one skilled in the art.

In another preferred embodiment of the process according to theinvention, a polymer is applied onto the material structure, morepreferably preceding consolidation of the material structure. Theadvantage of applying a polymer onto the materials structure beforeconsolidation is that it will at least partly impregnate the reinforcingelements, which leads to improved coherence of the material structureand thereby to improved durability.

The invention will now be further elucidated by the following figures,without however being limited thereto.

FIG. 1 a schematically represents a cross section of an apparatus formaking a valve prosthesis according to the invention, with the apparatusin opened configuration;

FIG. 1 b schematically represents a cross section of an apparatus formaking a valve prosthesis according to the invention, with the apparatusin closed configuration;

FIG. 2 schematically represents a detailed view in perspective of theapparatus of FIG. 1 a;

FIG. 3 schematically represents a hard mould, representing the innervolume of a heart valve prosthesis according to the invention;

FIG. 4 schematically represents a semi-finished heart valve prosthesisaccording to the invention, manufactured using the apparatus of FIG. 1and the hard mould of FIG. 3;

FIG. 5 a schematically represents a heart valve prosthesis according tothe invention with the leaflets in an almost opened position; and

FIG. 5 b schematically represents a heart valve prosthesis according tothe invention with the leaflets in a closed position.

Referring to FIG. 1 a, an example of an apparatus 1 for manufacturing avalve prosthesis according to the invention is shown. The apparatus 1comprises a box like holder 2, provided with a closure 4. Within holder2, an inner mould 10, made of hard steel, and defining the inner shapeof the valve to be produced, is provided. Around inner mould 10, arubber mould (8, 12) is provided. Holder 2 and closure 4 enclose rubbermould (8, 12) to pressurize it. Holder 2 comprises steel walls 3,provided with integrated heating elements (not shown). Closure 4 is alsomade of steel and provided with integrated heating elements (not shown).Closure 4 is dimensioned such that it partly fits into the inner space 5of holder 2. Bolt holes 6 are provided in closure 4 to match the innerscrew thread 7 of the steel walls 3 of holder 2, and attach closure 4 toholder 2. Lower rubber mould halve 8 is positioned on the bottom 9 ofthe inner space 5 of holder 2. An upper mould halve 12 is positionedover lower mould halve 8. Both rubber mould halves (8, 12) enclose hardmould 10, which is provided with a hollow braid 11 as materialstructure.

Referring now to FIG. 1 b, the rubber moulds (8, 12) are pressurized bylowering closure 4 into inner space 5 of holder 2, until it contacts theupper mould halve 12. Bolts 13 are placed through bolt holes 6 ofclosure 4 and partly positioned into the screw thread 7 of the steelwalls 3 of holder 2.

In a preferred embodiment, lower and upper mould halves (8,12) aremanufactured as follows. Holder 2 is filled about half way (see FIG. 2)with a two component curable silicone rubber resin Silastic® MRTVsilicone rubber from Dow Corning. Hard mould 10 is then placed half wayin the still uncured rubber of the lower mould halve 8 by suspending itfrom a thermocouple (not shown), connected to the walls 3 of the holder2. The hard mould 10 was treated with a release agent. After curing thelower mould halve 8, the hard mould 10 is removed from lower mould halve8 and replaced again. Another layer of release agent is then applied.Subsequently, an additional portion of rubber mixture is applied on topof lower mould have 8 with the hard mould 10 positioned onto the lowermould halve 8 to form the upper mould halve 12. Sufficient rubber isapplied to ensure that closure 4, when inserted into the inner space 5of holder 2, exerts enough pressure onto the lower and upper mouldhalves (8,12). After curing the upper mould halve 12, the lower andupper mould halves (8,12), as well as the hard mould 10 are taken apartand all rests of release agent removed. If desired, the rubber cavitiesin the lower and upper mould halves (8,12), as formed by the hard mould10, can be extended by cutting away some material, for instance with aknife.

Various valve designs are known in the art. In the following sectionsone preferred embodiment is described in details, however, based on thisthe skilled person would be able to adjust known manufacturing methodsto that of the present invention.

In one way of producing a valve according to the invention, the lowermould halve 8 is placed onto the bottom 9 of the holder 2. A hollowbraid 11 with a diameter of 2 centimetre and strands (110, 111) orientedat ±45° angle with the longitudinal axis, which strands are made fromcommercial Dyneema® SK75 fibers is pulled over the hard mould 10, asshown in FIGS. 1 and 3. Reinforcing elements in the form of strands(110, 111) extend in two directions. A first series 110 of strandsextends at an angle of about 90° with respect to a second series ofstrands 111. The inner mould 10 defines the inner shape of the valve tobe produced. In FIG. 3, the hard mould 10 is covered by a partiallyshown braid 11. The hard mould comprises a tubular part 20 and astar-shaped part 21 with a longitudinal central axis 22. The star shapedpart 21 comprises three surfaces 23, which are curved towards thelongitudinal central axis 22. The three surfaces 23 of the mould 10 formthe support for the leaflets of the valve prosthesis duringmanufacturing thereof. The tubular part 20 of the hard mould 10 extendsbeyond the dimensions of the valve prosthesis in its final form, toallow for easy positioning of the braid 11. About 2 mm excess materialat both sides of the mould 10 is left. A Mylar (polyethyleneterephthalate) film (not shown) is then placed over the braid 11. Theassembly of hard mould 10, covered by the braid 11, and the Mylar filmis then placed in the mould cavity of the lower mould halve 8.Subsequently, the upper mould halve 12 and the closure 4 are placed ontop, such that closure 4 makes contact with the upper mould halve 12.Bolts 13 are applied and tightened such that an estimated pressureexceeding 30 MPa is applied on the mould halves (8,12). After tighteningthe bolts 13, the heating system is switched on. The heating system iscontrolled such that a temperature of 145° C. occurs in the hard mould10. The temperature in the hard mould 10 is measured with athermocouple. Heating is switched off when the hard mould 10 issubjected to a temperature of 145° C. during one hour. The completeapparatus including the valve is allowed to cool down during a day andwas then opened. The front and back sides of the moulded valveprosthesis is then edge trimmed carefully, removing any excess material.Edge trimming is done with the hard mould 10 still inside, using a verysharp razor blade, making sawing movements. Sharpness and sawingmovements are necessary in view of the cut resistance of the Dyneema®SK75 fibers. Finally, the valve prosthesis is demoulded by pulling itaway from the hard mould 10. During the above described mouldingprocess, the individual Dyneema® fibers were fused together in the shapeof a valve prosthesis.

In a preferred embodiment, the shaping of the material structure in themould is conducted mainly or (preferably) solely by creep forming. Thisreduces the risk of formation of ripple in the structure and henceprovides a more uniform structure and smooth surface than traditionalforming processes where ripples may be formed.

Another embodiment of the invented valve was made, using a similarprocedure as described above, the difference being that a low densitypolyethylene film with a thickness of 10 micrometer was applied aroundthe braid 11, before covering it with a Mylar® film. Temperature was setat 135° C., being sufficient to melt the low density polyethylene film,thereby impregnating the fibers of the braid 11.

After demoulding, any excess material of the tubular part 20 is removed,and a support structure attached thereto. Referring to FIG. 5 a, thevalve prosthesis 40 comprises a ring-shaped support structure 43, madefrom Nitinol®, which is a commercially available nickel-titanium alloy.The valve prosthesis 40 furthermore comprises three leaflets 44, whichare an integrated part nearby the support structure 43, as they areshaped from the braid 11. The braid 11 is fixed to the support structure43 by sutures (not shown).

A semi-finished heart valve prosthesis 30 according to the invention andmanufactured using the device 1 of FIGS. 1 a and 1 b as well as the hardmould 10 of FIG. 3 is shown in FIG. 4. The semi-finished heart valveprosthesis 30 comprises a tubular part 31 and a star shaped part 32,which are integrally formed by the braid 11 with strands (110,111). Thestar shaped part 32 comprises three surfaces 33, which are curvedtowards the longitudinal central axis 34, which surfaces 33 form theleaflets of the semi-finished heart valve prosthesis 30. Thesemi-finished heart valve prosthesis 30 may need further processingsteps, such as shortening of the tubular part 31 as well as adding asupport structure.

FIGS. 5 a and 5 b show a heart valve prosthesis 40 according to theinvention, manufactured from the semi-finished prosthesis of FIG. 4. Theheart valve prosthesis 40 is obtained by shortening the tubular part 31of the semi-finished heart valve prosthesis 30, for example by using ahot knife, and by fixing a support structure 43 to the shortened end ofthe tubular part 41. The valve prosthesis 40 comprises a ring-shapedsupport structure 43, for example made from Nitinol®, which is acommercially available nickel-titanium alloy. The valve prosthesis 40furthermore comprises three leaflets 44, which are integrally part ofthe valve connected to themselves nearby the support structure 43, asthey are shaped from the braid 11. The braid 11 is fixed to the supportstructure 31 by sutures (not shown). It is obvious to the skilled personthat other numbers of leaflets 44, such as 1, 2, 4, 5, 6 or even more,may also be used (not shown in the figures).

In fact this valve prosthesis 40 is a tube with collapsed walls in atricuspid shape, forming the star shaped part 42. The leaflets 44 easilyopen with a light pressure towards the end. However, pressure in theother direction will cause additional closing pressure, thus preventingregurgitation.

In FIG. 5 a, the leaflets 44 are depicted in open position, as mediumflows in the direction according to the arrow A1.

FIG. 5 b shows the valve prosthesis 30 of FIG. 5 a, but now with theleaflets 44 in a closed position. This is the result of medium flowingin the direction according to the arrow A2 towards the valve prosthesis40. As the leaflets 44 of FIG. 5 a are configured such that theyslightly extend from the support structure towards a centrallongitudinal axis 34 of the semi-finished heart valve prosthesis 30 ofFIG. 4, a small pressure difference as a result of medium flowing in thedirection according to arrow A2, will press the leaflets 44 towards eachother, thereby closing the valve prosthesis 40 and preventing the mediumto flow through the valve prosthesis 40 according to arrow A2.

To aid in avoiding regurgitation, the valves are preferably produced ina shape that conforms to the closed position, because a slight pressurein the desired flow direction will cause opening of this embodiment,while the closed position will be approached rapidly if the pressure isreversed.

1. Implantable valve prosthesis (40), comprising at least one valveleaflet (44) and made of a material structure of unidirectionalreinforcing elements of drawn ultra high molecular weight polyolefin(11, 110, 111), extending in at least two directions, wherein themodulus of elasticity of the polyolefin reinforcing elements is at least60 GPa.
 2. Valve prosthesis according to claim 1, further comprising asupport structure (31, 43) for the at least one valve leaflet.
 3. Valveprosthesis according to claim 1, wherein the thickness of thereinforcing elements (11, 110, 111) is less than 40 micrometer, morepreferably less than 30 micrometer and most preferably less than 20micrometer.
 4. Valve prosthesis according to claim 1, wherein thematerial structure comprises at least one woven fabric or knitted braid(11) of the reinforcing elements (110, 111) of drawn ultra highmolecular weight polyolefin, preferably the material structure consistof one woven fabric or knitted braid (11) of the reinforcing elements(110, 111) of drawn ultra high molecular weight polyolefin.
 5. Valveprosthesis according to claim 1, wherein the material structurecomprises a plurality of superimposed multilayered material sheets (11),the sheets comprising a consolidated stack of monolayers of thereinforcing elements of drawn ultra high molecular weight polyolefin,whereby the draw direction of two subsequent monolayers in the stackdiffers.
 6. Valve prosthesis according to claim 1, wherein thereinforcing elements (110, 111) comprise fibers and/or tapes.
 7. Valveprosthesis according to claim 1, wherein the valve prosthesis isessentially free of matrix binder.
 8. Valve prosthesis according toclaim 1, wherein the material structure (11) comprises a plurality ofreinforcing elements (110, 111) embedded in a matrix binder.
 9. Valveprosthesis according to claim 1, wherein the polyolefin comprisespolyethylene, preferably the polyolefin comprises UHMWPE and morepreferably the polyolefin is substantially UHMWPE.
 10. Valve prosthesisaccording to claim 1, wherein the thickness of the material structure isbetween 50 and 500 micrometer, preferably between 80 and 400 micrometerand most preferably between 150 and 250 micrometer.
 11. Valve prosthesisaccording to claim 1, wherein the at least one valve leaflet (44) isintegrally formed with at least a part of the support structure (31,43).
 12. Valve prosthesis according to claim 1, wherein the supportstructure (43) is at least partly made from shape memory alloy, or fromsuperelastic alloy.
 13. Method of manufacturing an implantable valveprosthesis (40), comprising the steps of: providing a material structure(11) of unidirectional reinforcing elements (110, 111) of drawn ultrahigh molecular weight polyolefin, extending in at least two directions,wherein the modulus of elasticity of the polyolefin reinforcing elementsis at least 60 GPa; shaping the material structure (11) in the form ofat least a part of the valve prosthesis (40); and consolidating thematerial structure under elevated temperature and pressure.
 14. Methodaccording to claim 14, wherein a matrix binder is applied onto thematerial structure and consolidated with it under elevated temperatureand pressure.
 15. Valve prosthesis according to claim 1, the valveprosthesis being a heart valve prosthesis.